Lead frames are usually manufactured in long strips of many individual units. The long strips may be wide enough to accommodate from one to five lead frames. A lead frame may be equipped with carrier rails and guide holes to position the lead frames during manufacture. The lead frames may be comprised of a plurality of leads having lead ends and lead fingers, tie bars, and a die pad. The die pad is centrally located on the lead frame and provides the area on which a semiconductor chip may be mounted. When a strip is more than one lead frame wide, the strip is referred to as a matrix.
Most IC devices are assembled around a lead frame. Conventional lead frame plating processes produce lead frames with clean, non-reactive finishes of silver, palladium, nickel, or copper. The die pad of such a lead frame is downset during packaging and a chip, such as a silicon wafer or a semiconductor chip, is affixed to the die pad. Connection areas on the chip, e.g., bond pads, are connected to the lead frame's lead fingers by wire bonds. Excess lead frame material, such as the carrier rails, may then be trimmed away and the die pad, chip, leads, and associated connections are encapsulated in a plastic mold compound.
A source of failure in semiconductor IC packages is the delamination of the plastic mold compound from the lead frame. This may result from various causes including improper curing of the plastic mold compound and "popcorn failure." Popcorn failure may occur when liquids, such as water, are trapped beneath the encapsulation during packaging of the semiconductor devices or when liquids seep or vapors condense under the plastic mold. Moreover, plastic mold compounds are naturally capable of absorbing liquids from their environment until saturation occurs. Such liquids may be vaporized during semiconductor IC operation, and the expanding vapor, e.g., steam, may cause pressure to build up beneath the encapsulation. This pressure build up may cause a catastrophic mold failure.
Encapsulation failure may also result from thermal mismatch between dissimilar device materials, such as between the chip and the die pad, at solder reflow temperatures, e.g., temperatures in a range of about 419.degree. to 464.degree. F. (215.degree. to 240.degree. C.). Such mismatches may be aggravated by the pressure created by the heating of liquids absorbed by the plastic mold compound. The combined effect of these stresses degrades adhesion, leads to delamination--especially between the underside of the die pad and the plastic mold compound, and may cause encapsulation cracking or failure.
Efforts to resolve these problems have involved choosing and developing plastic mold compounds that improve adhesion between the die pad and the plastic mold compound. Encapsulations that are lower stress, stronger, absorb less moisture, and possess better adhesion capabilities have been proposed. Nevertheless, these have not eliminated the problems. Further, because specialized encapsulations are generally more expensive, their use tends to increase the costs of manufacturing.